gpsd: User/group/permission issues, does not work on USB serial GPS sources without manual chmod

Is this a new report?

Yes

System Info

Void 5.19.17_1 x86_64 GenuineIntel uptodate rFFFFFFFFFFF

Package(s) Affected

gpsd-3.24_2

Does a report exist for this bug with the project's home (upstream) and/or another distro?

No response

Expected behaviour

1. Plug in USB GPS source (mine is a u-blox 7)
2. Verify it works and has a GPS lock using gpsmon as root. This tool directly talks to the GPS device rather than needing gpsd to be working.

# gpsmon /dev/ttyACM0

3. Stop gpsmon

4. Start gpsd daemon in one terminal (as root, with verbose debugging flags like this)

# gpsd -N -F /run/gpsd.sock -D5 /dev/ttyACM0

5. Run cgps (or any other gps client) in another terminal. This will talk to gpsd and (if everything is working) give you similar output to what gpsmon did.

$ cgps

Actual behaviour

cgps fails to see any data and gpsd spits the following errors:

gpsd:PROG: CORE: gpsd_open(/dev/ttyACM0) fd -1
gpsd:PROG: SER: gpsd_serial_open(/dev/ttyACM0) sourcetype 12 fd -1
gpsd:INFO: SER: opening GPS data source type 12 at '/dev/ttyACM0'
gpsd:ERROR: SER: device open of /dev/ttyACM0 failed: Permission denied(13) - retrying read-only
gpsd:ERROR: SER: read-only device open of /dev/ttyACM0 failed: Permission denied(13)
gpsd:ERROR: /dev/ttyACM0: device activation failed, freeing device.

Looking at the running process list reveals that the gpsd process has changed to the 'gpsd' user.

Permissions for the serial device:

$ ls -lh /dev/ttyACM0 
crw-rw---- 1 root dialout 166, 0 Nov 30 22:38 /dev/ttyACM0

Adding the gpsd user to the dialout group does NOT fix the problem.

Allowing anyone rw access to the serial device DOES fix the problem temporarily

# chmod a+x /dev/ttyACM0

According to the gpsd manpage:

  Before doing any processing of GPS data, it tries to drop root
  privileges by setting its UID to "nobody" (or another configured
  userid) and its group ID to the group of the initial GPS passed on the
  command line — or, if that device doesn’t exist, to the group of
  /dev/ttyS0.

I do not understand how processes can have group ids, but regardless this feature does not seem to be working as intended.

The srcpkg template has this line in it, not sure if it is related to this problem or not:

make_build_args="dbus_export=0 gpsd_user=gpsd gpsd_group=gpsd

Steps to reproduce

See above.

Start with the gpsd privilege-drop behavior described in the manpage and the srcpkg template's make_build_args. Reproduce the issue with gpsmon, gpsd, and cgps, then compare the daemon's resulting user and groups with the /dev/ttyACM0 dialout permissions. Done means gpsd can open the USB serial device without manual chmod while still dropping privileges.